In a semiconductor device that is incorporated in an electronic computer or the like, such as a personal computer, and that performs predetermined processing, a volatile storage circuit has been provided according to need, so that the processing is performed while information necessary for the processing is sequentially stored into the volatile storage circuit.
Such a volatile storage circuit stores information by supplying electric power and has a characteristic of high writing and reading speed. In contrast, the volatile storage circuit has a characteristic of losing stored information when a power supply is interrupted due to a power-off operation, a sudden power failure, or the like. Thus, when a power source is turned on again after the interruption of the power supply, information stored before the interruption of the power supply does not exist in the volatile storage circuit. Therefore, information stored before the interruption of the power supply cannot be recovered.
Recently, an instant-on technology has been suggested. In the instant-on technology, a storage circuit includes a volatile storage circuit and a nonvolatile storage circuit that are connected in parallel to each other, and information equal to that stored in the volatile storage circuit is stored into the nonvolatile storage circuit. Thus, when a power supply is interrupted due to a power-off operation, a sudden power failure, or the like, necessary information is stored into the nonvolatile storage circuit. Then, when the power supply resumes, the state before the interruption of the power supply can be immediately recovered by using the information stored in the nonvolatile storage device.
However, in the above-mentioned storage circuit, even if information that has already been stored in the nonvolatile storage circuit is equal to information stored in the volatile storage circuit, writing processing of writing the information into the nonvolatile storage circuit must be performed. Thus, unnecessary power is consumed.
In particular, due to the nonvolatility, nonvolatile storage circuits require a lot of electric energy to write storage information. This is a factor preventing a reduction in power consumption.